Recently, various digital recording and reproducing apparatus for television signal which record a television signal on a recording medium and reproduce from it by sampling and converting it to digital signals have come to be proposed.
One kind of such digital recording and reproducing apparatus is business use one which mainly uses the characteristic of no-deterioration of the picture quality in case of dubbing. In this kind of business use apparatus, for instance, an NTSC color television signal is sampled at a frequency three times that of a color subcarrier f.sub.sc, and this sampled signal is directly subjected to PCM (pulse code modulation), without band compressing process, to be recorded and reproduced. When this NTSC color television signal is sampled at a frequency of 3.multidot.f.sub.sc and coded into 8 bits per sampling point, the recording bit rate becomes as high as 86 Mbits/sec.
Another kind of such digital recording and reproducing apparatus is one which is designed to record and reproduce a television signal digitally by using a home use VTR of low cost and low bit rate while somewhat sacrificing the picture quality. In a low bit rate digital recording and reproducing apparatus, it is necessary to use the band compression techniques to thereby reduce the recording bit rate.
Regarding the band compression, supposing that upper limit frequency of band of a signal to be band-compressed is f.sub.c, an original signal, generally, cannot be reproduced unless it is sampled by a frequency higher than 2.multidot.f.sub.c (which is called a Nyquist frequency). However, with respect to a signal whose frequency spectrum has a special configuration, such as television signals, it is known as the sub-Nyquist sampling method that an origianl signal can be reproduced almost completely by interpolation, even if the signal is sampled at a frequency lower than the Nyquist frequency of 2.multidot.f.sub.c by effectively making use of the special configuration of the spectrum. For instance, an NTSC color television signal can be sampled at a sub-Nyquist frequency of 2.multidot.f.sub.c (f.sub.sc is the frequency of color subcarrier). In this case, considering of coding into 8 bits per sampling point, the recording bit rate is 57.6 Mbits/sec, so that the recording bit rate of 86 Mbits/sec in the above case of sampling by three times may be decreased. But this rate of 57.6 Mbits/sec is still too high to record in a low bit rate machine such as a home use VTR.
In the past, it was attempted to reduce the recording bit rate to 28.6 Mbits/sec by use of the combination of 2H type sub-Nyquist sampling method and DPCM (differential PCM) coding. This attempt was disclosed in the article entitled "Experiment of low bit rate digital VTR", pages 576 .about.580 published in the Journal of the Institute of Television Engineering, Vol. 35, No. 7 (1981).
Incidentally, an encoder for DPCM coding and a decoder for demodulating DPCM coded signals are generally described below. First at the encoder side, a signal predicted by a predictor, and an input signal, of for instance, 8-bit, are compared and their error signal, i.e., a signed representative of difference between the above two signals is determined at a subtractor. The number of bits of the output signal of this subtractor is reduced, say, to 4 bits by a quantizer. At the same time, in a local decoder comprising an adder and an inverse quantizer having the inverse characteristics of the quantizer, a local decoded signal is obtained. The local decoded signal is applied to the predictor to obtain a prediction signal with respect to the present input signal. This prediction signal is applied to one input of the subtractor in order to obtain the error signal. On the other hand, at the decoder side, the above 4-bit signal from the encoder is first applied to another inverse quantizer having the same characteristics as the inverse quantizer in the encoder and is returned to the original 8-bit signal. In consequence, this 8-bit signal is fed to another adder, which finds the sum of this 8-bit signal and an output of another predictor having the same prediction characteristics as the predictor in the encoder so as to restore the input signal. Thus, in the process of decoding the DPCM coded signals, since the transmitted prediction errors are sequentially added on the basis of the previous value, if a bit error occurs in the midst of transmission, this error is propagated successively. That is, in the above case of recording and reproducing television signals by combining the sub-Nyquist sampling method and DPCM coding, although the recording bit rate may be reduced to a recordable region, there seems to be left a room for further improvement of the error propagation characteristics.
Meanwhile, in this DPCM coding, in order to minimize the difference between input signal and prediction signal, that is, so-called prediction error, and to reduce the number of quantizing bits of a quantizer, it is preferable to compose the prediction signal of the predictor from the signals at the sampling points arrayed in two dimensions on the television screen, or more preferably, in three dimensions including the direction of the time. However, in such two- or three-dimensional predictor, the error propagation also expands two-dimensionally or three-dimensionally. When the signal containing such error is reproduced, it results in an extreme deterioration of picture quality. At the present, however, there has been found no effective means of preventing the error propagation completely. It is possible to reduce the error propagation by correcting and concealing the error, but a larger circuit and a higher cost are required for this purpose. In particular, this problem of error propagation is fatal in a digital recording and reproducing apparatus with relatively high bit error rate of 10.sup.-4 .about.10.sup.-5 of reproduction signal such as a digital VTR for high density recording.
Accordingly, it is a principal object of the present invention to obtain a sufficinet reproduction picture quality in a recording and reproducing apparatus with a relatively high bit error rate such as a home use VTR by reducing the recording bit rate to a level recordable even by a home-use VTR and simultaneously minimizing the bit error propagation range while maintaining the picture quality of the input television signals.
This and other objects are accomplished by a digital recording and reproducing apparatus for television signal comprising sampling means for sampling a television signal with upper limit frequency f.sub.c at a frequency lower than 2.multidot.f.sub.c and to deliver a signal quantized into N bits, orthogonal transformation means for transforming orthogonally to a block composed of L sampling points out of the above quantized sampling points, quantizing means for quantizing the orthogonally transformed signals into average M bits (M .ltoreq.N) per sampling point, recording means for recording the television signals obtained by quantizing onto a recording medium, reproduction means for reproducing the recorded television signals out of the same recording medium, inverse quantizing means having the inverse quantizing characteristics of the above quantizing means for quantizing inversely the television signals reproduced by the reproduction means, orthogonal inverse transformation means having the inverse transformation characteristics of the above orthogonal transformation means for inversely transforming orthogonally the above inversely quantized signals, and interpolation reproduction means for synthesizing an interpolation signal from a sampling point adjacent to this inversely orthogonally transformed television signal and for interpolating the above reproduction signal by the interpolation signal.
In the embodiments of this invention, the television signal is component color television singal or composite color television signal. The sampling frequency is an integer multiple of horizontal scanning frequency f.sub.H of a television signal. The orthogonal transformation is an Hadamard transformation. The block of orthogonal transformation is composed of L sampling points adjoining to each other arrayed in a rectangular grid form or a rectangular body grid form two-dimensionally or three-dimensionally. The block of orthogonal transformation may be composed of L sampling points adjoining to each other arrayed two-dimensionally within a present field. Furthermore, the block of orthogonal transformation may be composed of L sampling points adjoining to each other arrayed three-dimensionally across plural fields or frames. As the block of this orthogonal transformation, a first block composed of L sampling points adjoining to each other arrayed two-dimensionally within a field, and a second block composed of L sampling points adjoining to each other arrayed three-dimensionally cross plural fields or frames are selectively used depending on suitability. The aforesaid composite color television signal is an NTSC color television signal, and its sampling frequency is twice the color subcarrier freqeuncy f.sub.sc.
In other embodiments, it is constituted that the television signal with upper limit frequency f.sub.c be sampled with a frequency which is lower than 2.multidot.f.sub.c and an integer multiple of horizontal scanning frequency f.sub.H of television signal and shifting the phase by 180.degree. in every field. It is also constituted to synthesize an interpolation signal from adjacent sampling points of at least one field before as for the reversely orthogonally transformed television signal. The block of orthogonal transformation is composed of L sampling points adjoining to each other arrayed in a rectangular grid form within a field. The interpolation reproduction means is constituted so that the higher frequency portion of the recorded television signals may be interpolated from the adjacent sampling points of one field before while the lower frequency portion may be done from the adjacent sampling points in the present field.
Another embodiment comprises movement detecting means for detecting the time-related movement of reproduction signals, first interpolation signal synthesizing means for synthesizing an interpolation signal from adjacent sampling points within a present field, second interpolation signal synthesizing means for synthesizing an interpolation signal from adjacent sampling points of at least one field before, interpolation signal selecting means for selecting one of the outputs from first and second interpolation synthesizing means according to the information from the movement detecting means, and interpolation processing means for interpolating the reproduction signals by the interpolation signal selected by the interpolation signal selecting means.
Still more, this invention is applied to a digital recording apparatus for television signal comprising sub-Nyquist sampling means for sampling a television signal with upper limit frequency f.sub.c at a frequency lower than 2.multidot.f.sub.c, orthogonal transformation means for assembling the sampled sampling points into a block and orthogonally transforming to the block, quantizing means for quantizing the orthogonally transformed signal, and recording means for recording the television signal obtained by this quantizing onto a recording medium. In this case, the sampling frequency is 2.multidot.f.sub.sc (f.sub.sc is a color subcarrier frequency).
More specifically, this invention relates to a digital recording and reproducing apparatus for television signal intended to sample the television signal with upper limit frequency f.sub.c at a frequency lower than 2.multidot.f.sub.c, assemble the sampled sampling points into a block, orthogonally transform to the block, quantize the orthogonally transfomred signals, and reproduce the signals recorded onto a recording medium, comprising reproduction means for reproducing the recorded television signals, inverse quantizing means having the inverse quantizing characteristics of the quantizing in recording mode for inversely quantizing the television signals reproduced by the reproduction means, orthogonal inverse transformation means having the inverse transformation characteristics of the orthogonal transformation in recording mode for inversely transforming orthogonally the inversely quantized signals, and interpolation reproduction means for synthesizing an interpolation signal from adjacent sampling points with respect to the inversely orthogonally transformed television signals for interpolating the reproduction signals by the interpolation signal.
As explained above, according to the digital recording and reproducing apparatus for television signal of the present invention, the following effects will be obtained.
(1) An input television signal can be recorded at a low bit rate (for instance 28.6 Mbits/sec), and at the same time, as compared with the case using DPCM coding, the error propagation characteristic is extremely improved. Therefore, the load to error correction and error concealment is smaller, and the circuit may be reduced in size and the cost is also lowered. Even when a home-use VTR is used, a television signal can be recorded and reproduced digitally.
(2) In case an incorrectable error occurs, it does not cause a serious problem visually, and a sufficient reproduction picture quality for home viewing may be obtained, since its propagation range is limited to a narrow area.
While the novel features of the invention are set forth with particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description in conjunction with the drawings.